The barrier function of intestinal epithelial cells is mediated largely by tight junctions between adjacent cells. Tight junctional permeability is regulated by a number of disparate factors including several inflammatory mediators, by processes which involve changes in the cellular cytoskeleton. There is accumulating evidence that increased junctional permeability could play an important role in the etiology, progression and maintenance of chronic diarrheal and inflammatory bowel diseases. In particular, an inherited disorder in junctional permeability may predispose individuals to Crohn's disease. In order to understand the molecular basis for increased junctional permeability, a better basic understanding is needed of the physical nature of this paracellular pathway and the mechanisms and factors which regulate its permeability. This application will identify the role and mechanism of insulin and insulin-like growth factors (IGFs) in regulation of junctional permeability. Using monolayer cultures of the differentiated human colonic epithelial cell line, T84, we have shown that insulin and IGFs cause a specific, reversible and dose dependent increase in junctional permeability. Using monolayer cultures of the differentiated human colonic epithelial cell line, T84, we have shown that insulin and IGFs cause a specific, reversible and dose dependent increase in junctional permeability. Using this model system, we will investigate how these peptide hormones regulate permeability by 1) identifying the receptors which mediate the effect; 2) identifying intracellular mechanisms involved in transducing the response; and 3) identifying changes in the cellular cytoskeleton which accompany changes in permeability. We will then determine whether autocrine production of IGF-like peptides accounts for the inability of T84 cells to retain high transepithelial resistance when grown in defined, serum-free media. Completion of this aim will result in defined, serum-free media conditions useful in examining the production and response of cultured epithelial cells to other growth factors and cytokines. Finally, we will extend our observations to non-tumorigenic cell lines in culture and to tissues from rabbits with experimentally induced colitis to determine whether insulin-like growth factors are likely to mediate increased mucosal permeability in vivo. These experiments are likely to elucidate the factors and mechanisms involved in regulating junctional permeability. This could lead to a more rational approach in understanding the etiology and in improving treatment for inflammatory bowel disease.